CO\(_2\)  is the main form of carbon in thermochemical equilibrium with H\(_2\) O [1]}, [2]}. If a liquid-water ocean exists, the partial pressure of CO\(_2\)  is set by atmosphere-ocean partitioning, which in turn is mainly controlled by the oceanic pH [3]}, [4]}, [5]}, [6]}. The pH is affected by the abundance of cations in the ocean, which come from complex water-rock reactions and dissolution of the seafloor. The rates of the processes involved are uncertain; therefore, we explore the mixing ratio of CO\(_2\)  from 400 ppm to 10%, corresponding to the pCO\(_2\)  range from the present-day Earth to early Earth [7]} and including the predicted range for ocean planets [5]} that is still consistent with an H\(_2\) -dominated atmosphere. The \(4\times 10^{-4}\)  bar partial pressure of CO2 in the low-CO\(_2\)  case, while not the absolute lower limit, is a cosmochemically and geologically reasonable lower bound of the CO2 partial pressure on an ocean planet (Appendix ).
